René Descartes' illustration of brain anatomy and physiology. Inputs are passed on by the sensory organs to the pineal gland in the brain and from there to the motor nerves. At the pineal gland, the nerve processes affect the mind, an immaterial spirit, in accordance with Descartes' mind/body dualism. The mind can also affect the pineal gland, thereby directing the processes of the motor nerves. From Descartes (1664).

The mind-body problem can be stated as, "What is the basic relationship between the mental and the physical?" For the sake of simplicity, we can state the problem in terms of mental and physical events: "What is the basic relationship between mental events and physical events?" It could also be stated in terms of the relation between mental and physical states and/or processes, or between the brain and consciousness.

There are three basic metaphysical positions: mental and physical events are totally different, and cannot be reduced to each other (dualism); mental events are to be reduced to physical events (materialism); and physical events are to be reduced to mental events (idealism). To put it in terms of what exists "ultimately," we could say that according to dualism, both mental and physical events exist ultimately; according to materialism, only physical events exist ultimately; and according to idealism, only mental events exist ultimately. Materialism and idealism are both varieties of monism, and of monism there are two further varieties, namely dual-aspect monism and neutral monism.

The absence of an empirically identifiable meeting point between the non-physical mind and the physically extended body has proven problematic to dualism and many modern philosophers of mind maintain that the mind is not something separate from the body.[1] This approach has been influential in the sciences, particularly in the fields of sociobiology, computer science, evolutionary psychology and the various neurosciences.[2][3][4][5]

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Dualism is the idea that the mental and the physical are two completely different kinds of things. In more technical language, dualism holds that mind and body are distinct types of substance, where a substance is a type of thing or entity that can exist on its own, independently of other types of entities. In traditional ontology, substances are the ultimate bearers of properties. They are definable by their essential properties, those properties that make them to be the kind of thing that they are. The essential properties of mind would thus be the mental properties, whichever they are (e.g., conscious states, states that are inherently representational, or however the mental is defined). Body or matter would have as its essential properties the physical or material properties, whichever they are (e.g., spatial extension, mass, force, or however the physical or the material is defined).

Within Western Philosophy, the first major proponent of dualism was Plato. Plato put forward a theory that the most basic realities are Forms or abstract types, a theory known as Platonic idealism. But he also held that mind and body are distinct. Subsequent Platonists, such as Augustine of Hippo, adopted this position.

The most famous adherent of dualism was Descartes, who proposed a type of dualism that has come to be known as Cartesian dualism or Interaction Dualism. Cartesian dualism is the idea that mind and body are two fundamentally different types of things, but that they can interact in the brain. Physical events can cause mental events—for example, the physical act of hitting your hand with a hammer can cause nerve processes that affect the mind and produce the experience of pain. Conversely, mental events can cause physical events—for example, the mental decision to speak can cause nerve processes that make your tongue move.

A dualistic philosophy, by definition, recognizes the existence of both mind and body. In Descartes' philosophy, the body plays an important role in psychological functions. This is most clearly seen in his theory of the passions, which is a "body-first" theory. That means that bodily mechanisms condition which passion or emotion a human being will feel in given circumstances. These bodily mechanisms direct the person's response to the situation: flight from a frightful animal, embrace of a friendly companion. The mind's role is then to continue or to redirect the response that has begun with the body.[6]

In the twentieth century, some interpreters concluded that Descartes' philosophy leads one to consider the corporeal as of little value[7] and trivial. A rejection of this type of view of the mind-body relation is found in French Structuralism, and is a position that generally characterized post-war French philosophy.[8]

Epiphenomenalism can be another type of dualism, if the epiphenomenalist holds that mind and body are two fundamentally different types of things. This substance epiphenomenalism agrees with Cartesian dualism in saying that physical causes can give rise to mental events-- the physical act of hitting your hand with a hammer will create the mental experiene of pain. Unlike Cartesian dualism, epiphenomenalism argues that mental events cannot under any circumstances give rise to physical effects. So, if my hand touches fire, the physical heat can cause the mental sensation of pain, and my hand instantly recoils. It might appear that the mental experience of pain caused the physical event of the hand pulling back. According to Epiphenomenalism, this is an illusion—in reality, the physical heat directly caused the recoiling of the hand through nerve processes, and these same processes also cause the sensation of the pain. The mental events are caused by physical events, but they cannot themselves have any affect on matter.

Parallelism, as a form of dualism, argues that mental and physical events occur in separate domains and constitute two fundamentally different types of things that can never interact in any way. This view admits that physical events appear to cause mental effects (hitting your hand with a hammer appears to cause pain) and that mental events appear to cause physical effects (deciding to speak appears to cause your tongue to move). Parallelism, however, holds that this correspondence between the mental world and the physical world is simply a correlation, not the result of causation. The nerve processes caused by the hammer form a close looped that cause your hand to draw back. A separate chain of mental events run in parallel; you see the hammer hit your hand, and you subsequently feel pain. In this view, the mental world and the physical world are parallel, but separate, never directly interacting.

Physicalism is the idea that everything in the universe can be explained by and consists of physical entities such as matter and energy. The most basic form of physicalism is the identity theory, according to which all mental states are identical with physical states in the brain. In this view, while mental entities (such as thoughts and feelings) might at first appear to be a completely novel type of thing, in reality, the mental is completely reducible to the physical. All your thoughts and experiences are simply physical processes in your brain. Physicalism argues that ultimately the physical world and its laws explain the behavior of everything in the universe, including the behavior of humans.[9] Physicalism is sometimes called materialism.

According to physicalism, when you hit your hand with the hammer, nerve processes proceed to the brain and cause a central brain state. This central brain state does not cause you to feel pain, rather it is your pain. Somehow, the pattern of activation of neurons in your brain just is the feeling of pain. For every type of mental state, there should be a corresponding physical state to which it reduces. Thus, your decision to speak is also simply another pattern of activation in the brain. This neural activity, which itself just is the decision, then causes your tongue to move.

The entire sequence of causation could be described solely in physical terms. But it may also be described using the mental or psycyhological terms that name states and processes that are identical to physical states and processes. Reductive physicalism does not eliminate the mental; rather, it reduces the mental to the physical. Another form of materialism, call "eliminative materialism," seeks to eliminate, rather than to reduce, mental predicates.

Well-known identity theorists of the twentieth-century included the British philosopher U. T. Place and the Australian philosopher J. J. C. Smart.

Idealism is the view that physical objects, properties, events (whatever is described as physical) are reducible to mental objects, properties, events. Ultimately, only mental objects exist. According to idealism, the material world is not unlike a dream. When you have a vivid dream, you find yourself in a dream world that appears to be composed of material objects. In reality, however, everything in your dreamworld is a creation of your dream. If you dream you are riding a bicycle, the bicycle certainly feels real. In reality, however, the bicycle does not have an independent existence outside of your own mind. When you awaken, the bicycle might cease to be. Idealism holds that the entire "real world" of our waking lives is fundamentally a mental creation. Only minds and their experiences exist.

The best known idealist is the eighteenth-century Irish philosopher George Berkeley, Bishop of Cloyne. Berkeley argued that the notion of material substance is incoherent. As a consequence of his argument, he concluded that only minds and their internal states, or "ideas," exist. He granted the existence of human minds and of a divine mind, or God. According to Berkeley, all the ideas in the human mind are produced by God. Sensory ideas are produced in the form of a coherent view of what appears to be a physical reality. But, he maintained, these sensory ideas in fact have only mental existence. All the shapes and colors that we experience exist only in the mind. Berkeley is responsible for the philosophical chestnut, "if a tree falls in the forest and no one is there, does it produce a sound?" He answered that it does, because the infinite mind, God, is aware of the tree and its sound.

Physicalism and idealism are substance monisms. They posit only one type of substance in the world, whether physical or mental. Another type of monism is dual-aspect monism. This position holds that there is only one type of substance, which is itself neither physical nor mental. Rather, the physical and the mental are two aspects of this substance, or two ways in which the one substance manifests itself. When you hit your hand with the hammer, the damage to your skin, muscles, and bones is the physical aspect, and the pain is the mental aspect. The damage doesn't cause the pain. Rather, the damage and the pain are two aspects of a single underlying state of the one substance. This view was made prominent by the 17th century Dutch philosopher Benedict de Spinoza.

Yet a further type of monism is called neutral monism. This view denies that the mental and the physical are two fundamentally different things. Rather, neutral monism claims the universe consists of only one kind of stuff, in the form of neutral elements that are in themselves neither mental nor physical. These neutral elements are like sensory experiences: they might have the properties of color and shape, just as we experience those properties. But these shaped and colored elements do not exist in a mind (considered as a substantial entity, whether dualistically or physicalistically); they exist on their own.

Some subset of these elements form individual minds: the subset of just the experiences that you have for the day, which are accordingly just so many neutral elements that follow upon one another, is your mind as it exists for that day. If instead you described the elements that would consistute the sensory experience of rock by the path, then those elements constitute that rock. They do so even if no one observes the rock. The neutral elements exist, and our minds are constituted by some subset of them, and that subset can also be seen to constitute a set of empirical observations of the objects in the world. All of this, however, is just a matter of grouping the neutral elements in one way or another, according to a physical or a psychological (mental) perspective.

Another philosophical viewpoint, known as New Mysterianism, holds that the solution to the mind-body problem is unsolvable, particularly by humans. Just as a mouse could never understand human speech, perhaps humans simply lack the capacity to understand the solution to the mind-body problem.[10]

Throughout the history of modern psychology, many theorists have been interested in the mind-brain relation, no matter which metaphysical position they adopted (dualism or one of the monisms). Descartes held that there are lawful relations between brain states at the pineal gland and sensations or felt emotions. During the nineteenth-century, Gustav Fechner developed psychophysics as a way to study the relations between the mental and the physical. He is best known for his "outer psychophysics," which seeks to describe the relation between external physical stimuli, such as lights of various wavelengths, and mental states, such as sensations of one or another color. He was, however, perhaps even more interested in "inner psychophysics," which would establish a relation between brain states and sensations.[11]

During the latter part of the nineteenth century, the new experimental psychologists often held that the empirical questions of scientific psychology should be posed in a way that did not presuppose any one of the metaphysical solutions to the mind–body problem. They thus sought to study the mind–brain relation, without seeking to establish scientifically whether, say, materialism or neutral monism was the better metaphysical theory. William James, who as a philosopher endorsed neutral monism, advised the psychologist to take the methodological attitude of "empirical parallelism."[12] By this he meant that the psychologist should simply seek to chart the empirical correlations between mental states and brain processes. James viewed this position as a "provisional halting place,"[13] pending further philosophical progress on the metaphysics of the mind–body problem.

During the twentieth century, psychologists adopted various outlooks on the mind–body problem. Behaviorists such as John B. Watson, Clark Hull, and B. F. Skinner, were eliminativists. They wished to do away with all mentalistic talk in scientific psychology. Other behaviorists, such as Edward C. Tolman, allowed for mentalistic talk in psychology, including talk of mental representations, goals, and purposes, as long as such notions were clearly tied to observable behavior. Tolman did not propose a solution to the mind–body problem, but neither did he follow the other behaviorists in ignoring mental factors.

The Gestalt psychologists, Wolfgang Kohler, Kurt Koffka, and Max Wertheimer, proposed their own account of the mind–body relation, in the form of psychophysical isomorphism. Gestalt psychology is noted for its emphasis on organized wholes within perceptual and cognitive experience. An example of such organized structure is the figure/ground relation as in the Rubin vase at the right of the image, or multi-stable structure, such as the Necker cube on the left.

Necker cube and Rubin vase. The Necker cube shifts between two differently oriented cubes in three-dimensions. The Rubin vase shifts between a vase and opposed faces.

According to Gestalt psychophysical isomorphism, the organization in perceptual experience is directly correlated with organized field structures in the brain. When the Necker cube shifts its structure, the field structures undergo a parallel shift. The Gestaltists did not hold that the brain process was literally shaped as a cube, but they did posit a strong spatial isomorphism. The brain process should have structures that correspond to the faces of the cube, and these structures should alter during the switching of the experienced Necker cube. Similarly for the faces and vase. The Gestaltists did not claim to solve the mind–body problem. Rather, they were proposing an explanatory relation between brain structures and correlated mental events (perceptual experiences). They did not ultimately decide whether the metaphysical relation was reductive or fit another theory, such as dual-aspect monism.

Throughout the twentieth century, some perceptual psychologists retained an interest in establishing the neural correlates of psychological processes. This sort of empirical interest was especially strong among perceptual psychologists working on color perception. Leo Hurvich and Dorothea Jameson, using phenomenally based methods of psychological research, revived the opponent-process theory of color perception.[14] Other experimentalists, using single-cell recording techniques, then found evidence for opponent neural responses in the retina[15] and in the lateral geniculate nucleus (LGN).[16]

During the 1980s, the perceptual psychologists Davida Teller and Ed Pugh attended to the formal structure of the problem of establishing psychoneural relations. They encouraged physiological psychologists to formulate explicit psychoneural linking propositions, that is, propositions relating dimensions of perceptual experience to specific processes and structures in the brain.[17]

In the neuroscience community outside of physiological psychology, for some decades prior to the 1990s few neuroscientists spoke of consciousness and even fewer would be bold enough to try to approach the topic scientifically. During the 1990s, a major shift occurred in this wider neuroscience community: the topic of consciousness and its relation to brain function was again accepted as a respectable topic that many neuroscientists took seriously. Because of the legacy of behaviorism, which was strong and long-lasting in neuroscience, consciousness had not been considered a topic that was amenable to the methods of science. The change in the neuroscientific community of the 1990s is largely due to outspoken scientists such as Nobel-laureatesFrancis Crick and Gerald Edelman, as well as the influence of philosophers such as David Chalmers, Daniel Dennett, and Fred Dretske.

Most neuroscientists believe in the identity of mind and brain, the position of materialism and physicalism. However, some neuroscientists may instead embrace dual-aspect monism, because they do not accept that in postulating an identity between mind and brain (or more specifically, particular types of neuronal interactions) they are implying that mental events are "nothing more" than physical events. Such a neuroscientist would accept that physical events and mental events are different aspects of a more fundamental mental–physical substratum which can be perceived as both mental and physical, depending on perspective. Working neuroscientists do not often address their metaphysical convictions in print, so it may be difficult to know which of these positions they hold (if either), or even whether they distinguish between them.

Since most neuroscientists believe in the identity of mind and brain, it is not surprising that the search for the neural correlate of consciousness (NCC) has become something of a Holy Grail in the neuroscientific community.

Neuroscience has not established an NCC. Future research will reveal how far it can go in discovering such correlates. However, discovering such correlates will not solve the mind–body problem as it is normally posed. To see this, consider that individual theorists with widely differing theoretical perspectives all would have liked to learn the exact empirical correlations between brain processes and conscious states. These theorists include Descartes, Fechner, and the Gestalt psychologists, who had differing outlooks on the mind–brain relation.

To solve the mind–body problem it will not be enough to show that perception or consciousness is correlated with neural processes. Rather, a theoretical solution would need to explain the experienced aspects of perception and consciousness by showing how such aspects can be derived from the activity of neurons (or whatever aspects of brain activity are relevant). No one has proved that such an understanding cannot be achieved (contrary to the dire predictions of the new mysterians). On the other hand, no one really knows what such an understanding would look like. That's why the mind–body problem remains today in the category of unsolved problems. Work toward a solution may be facilitated if theorists are explicit about their assumptions and goals in studying the relations between the mental and the physical.